In the Perl programming language, autovivification is the automatic creation of new arrays and hashes as required every time an undefined value is dereferenced. Perl autovivification allows a programmer to refer to a structured variable, and arbitrary sub-elements of that structured variable, without expressly declaring the existence of the variable and its complete structure beforehand.[1]
In contrast, other programming languages either: 1) require a programmer to expressly declare an entire variable structure before using or referring to any part of it; or 2) require a programmer to declare a part of a variable structure before referring to any part of it; or 3) create an assignment to a part of a variable before referring, assigning to or composing an expression that refers to any part of it.
Perl autovivification can be contrasted against languages such as Python, PHP, Ruby, and many of the C style languages, where dereferencing null or undefined values is not generally permitted. It can be compared to the HTML standard's "named access on the window object"[2] which results in corresponding globally scoped variables being automatically accessible to browser-based JavaScript.
It is important to remember that autovivification happens when an undefined value is dereferenced. An assignment is not necessary. The debugger session below illustrates autovivification of a hash just from examining it:
Perl 5.6.1 and newer support autovivification of file and directory handles.[3] Calling open on an undefined variable will set it to a filehandle. According to perl561delta, "[t]his largely eliminates the need for typeglobs when opening filehandles that must be passed around, as in the following example:
The C++ Standard Library's associative containers (std::unordered_map and std::map) use operator[] to get the value associated to a key. If there is nothing associated to this key, it will construct it and value initializethe value. For simple types like int or float, the value initialization will be zero initialization.
Another example of counting occurrences of strings:
A similar trick can be achieved with the insert method, which returns an iterator to the element associated to the key, even if it already exists.
Python's built-in dict class can be subclassed to implement autovivificious dictionaries simply by overriding the __missing__ method that was added to the class in Python v2.5.[4] There are other ways of implementing the behavior,[5] [6] but the following is one of the simplest and instances of the class print just like normal Python dictionary objects.
>>> # Common names by class, order, genus, and type-species>>> common_names = Tree>>> common_names['Mammalia']['Primates']['Homo']['H. sapiens'] = 'human being'>>> common_names
>>> # Famous quotes by play, act, scene, and page>>> quotes = Tree>>> quotes['Hamlet'][1][3][3] = 'This above all: to thine own self be true.'>>> quotes
Ruby hashes can take a block specifying an object to be returned for non-existing indexes. These can be used to implement autovivificious maps.
Java Map has a method computeIfAbsent[7] that can be used to emulate autovivificous maps.
public static void main(String[] args)
PHP arrays are natively autovivificious.
ES6 introduces a new class that can be used to implement autovivification. With other features of JavaScript, this can be reduced to a single line of code:
// Test:var t = tree;t.first.second.third = 'text';console.log(t.first.second.third); // or t['first']['second']['third']
C#, using indexers and C# 4.0 dynamics,
// Test:var t = new Tree;t["first"]["second"]["third"] = "text";Console.WriteLine(t["first"]["second"]["third"]);
DynamicObject can be used for implementing different syntaxes also,
class Tree : DynamicObject
// Test:dynamic t = new Tree;t.first.second.third = "text";Console.WriteLine(t.first.second.third);
// or,dynamic t = new Tree;t["first"]["second"]["third"] = "text";Console.WriteLine(t["first"]["second"]["third"]);